A conventional television picture tube comprises three principal parts: an electron emitting cathode, an envelope for containing the cathode, and a panel attached to the envelope through which the picture is displayed. The electron beam emitted from the cathode activates points or dots of fluorescent materials applied on the inside surface of the panel, thereby creating an image. This bombardment on the fluorescent materials by a stream of electrons causes the generation of harmful X-ray. Since transmission of X-ray through the glass panel creates a safety problem for the television viewer, the glass panel of a television tube is required to have the capability of shielding X-ray radiation below a regulated safety level.
Accordingly, there have been made many efforts to develop glass having a high X-ray absorption coefficient, and, as a result, panel glass products having a linear X-ray absorption coefficient of, e.g., 28 to 29 cm.sup.-1 at a wavelength of 0.6 .ANG. have become commercially available.
However, the advent of projection cathode ray tubes has required higher operating voltages which induce higher levels of X-ray emission. Further, commercial needs for lighter-weight panels with a reduced glass thickness have necessitated a higher capability, in the panels, of absorbing X-ray radiation. Accordingly, further efforts have been made to manufacture a panel glass having an even higher X-ray absorption coefficient. For example, glass compositions for a cathode ray tube panel having a linear X-ray absorption coefficient of at least 30 cm.sup.-1 at 0.6 .ANG. are described in U.S. Pat. Nos. 4,277,286, 4,830,990, 5,468,692, Japanese Patent Publication No. 2,525,737 and Japanese Laid-open Patent Publication No. 206466/1995. Disclosed in these patent documents are BaO and SrO as major X-ray absorbing materials and ZnO and ZrO.sub.2 as supplementary materials for shielding X-ray transmission.
However, use of increased amounts of such high X-ray absorbing materials, e.g., BaO, SrO, ZnO and ZrO.sub.2, tends to bias the color of the base glass towards green and yellow. Further, as the contents of BaO and SrO increase, so does the liquidus temperature of the glass, causing the problem of devitrification. Therefore, the glass compositions disclosed in the prior art may have the devitrification problem or do not satisfy the chromaticity required of a panel glass for cathode ray tubes.